Differentiated Services

The IP header contains an 8-bit field known as the Type of Service (TOS) field. The intent of this field was to allow IP to support different classes of service, allowing a consistent implementation of IP quality of service (QoS). Agreement about how these bits were set and interpreted across routing domains never materialized, and the TOS field languished.

The Diffserv working group redefines the IP TOS field into categorized differentiated services code points (DSCP), with consistent definitions of the bits in the TOS. The goal of Differentiated Services (Diffserv) is to classify and police packets at the edge of a network so they can be consistently marked. Then queuing and policing inside of the network can make decisions based on defined per-hop-behaviors.

A per hop behavior (PHB) is an agreement between devices implementing Diffserv that a given DSCP is treated the same way at each hop. For example, DSCP 00100000 receives priority treatment compared with DSCP 10000000. Or the first DSCP receives 25 percent of the link bandwidth guaranteed while the second DSCP receives 10 percent guaranteed. The PHB defines how one DSCP is treated relative to another in a given Diffserv device.

Some PHBs are defined by standards. Two are assured forwarding (AF) and expedited forwarding (EF). AF guarantees bandwidth to packets set with this DSCP, and EF reserves bandwidth and ensures the lowest delay and jitter for applications. Network administrators using one of the 64 code points defined by the Diffserv standards can define additional PHBs.

Diffserv is primarily a method of classification. Routers do not communicate with each other (signaling) to communicate resource needs for a DSCP. Diffserv is better-effort routing, because its method of prioritization allows routers to make more informed forwarding decisions than best-effort routing. The lack of signaling and resource reservation has made Diffserv popular in enterprise networks where all traffic entering the network can be policed and properly shaped. As with other policing and shaping technologies, Diffserv allows the queuing mechanisms on the backbone to work efficiently if proper engineering is done at the edge.